The present invention relates to a system for laying an underwater pipe line from an ice surface.
With the growing demand for oil and gas, it is becoming necessary to tap reserves in some of the more remote and environmentally inhospitable parts of the world. Considerable effort is being devoted to the task of tapping Arctic deposits and bringing the oil or gas to market by means of pipe lines. Certain proposals, for example exploiting oil and gas deposits in the Canadian High Arctic, necessitate the crossing of large stretches of water between islands. In addition, oil and gas reserves may also be located in off shore regions of the Arctic coast which will in turn necessitate underwater gathering pipeline systems to bring the petroleum reserves to central production facilities.
In high Arctic regions ice formation begins in the early fall, generally continues to build in thickness until approximately mid May and then begins to reduce in thickness until break up occurs in the early summer. The magnitude of ice thickness is extreme with average depth of ice being around 6 to 8 feet. Pressure ridges and rafting also occur so that the ice surface will be anything but smooth and may attain intermittent thickness of approximately 15 feet.
While the ice surface may appear to be substantial it is in effect a giant plastic medium which is subject to limits of loading from the surface and also responds to the tide effects that occur in the High Arctic regions. In addition to the limitations of the strengths of the ice the severity of the High Arctic climate will be a major factor in construction. In the high Arctic Island systems, winter temperatures of -70.degree. F. are not uncommon and wind velocities in excess of 70 m.p.h. may also occur at the same time.
The utilization of high capacity pipelines, that is large diameter pipelines, appears to be one of the most satisfactory means of transporting the vast reserves of oil and gas from the High Arctic to the marketing areas.
Additional research into the inter island passages through which a pipeline system may be constructed have recently revealed that channel depths of 800 to 1,200 feet and in some cases beyond that may be expected. To date in the North Sea, which is considered to be a very hostile area, pipe has been laid to depths of only 450 feet by conventional marine lay barges.
Clearly inter island pipelines and potential off-shore pipeline gathering systems to be constructed in the High Arctic Islands will be made under the most awesome conditions of extreme sub-zero temperatures and extreme high wind velocities, the combination of which may make visibility almost zero.
Historically, the most common method of installing marine pipelines has been by the lay barge method. In this method, relatively short pipe lengths are joined together on the barge, the barge is then moved forward and the assembled pipe is paid out at the stern of the barge and subsequently laid on the ocean floor. As the pipe exits from the barge it follows a somewhat S-shaped configuration from the barge deck to the ocean floor. This S-shaped configuration may be generally divided into two regions, the overbend region and the sag bend region. The overbend region by definition is that portion of the said S-shaped configuration located immmediately behind the stern of the barge and follows a rearwardly and downwardly convex pattern that will terminate at the point of counterflexure. The sag bend region is by definition that portion of the said S-shaped configuration from the point of counterflexure rearwardly and downwardly in a concave manner to the relative touchdown position of the pipe on the bottom contour. The point of counterflexure is generally described as that position where the overbend region and the sag bend region join together.
Stresses imposed on the pipe in the overbend region are normally controlled by means of an elongated stinger connected to the barge, over which the pipe passes. As the pipeline is paid out from the stern of the barge into the stinger, which may be buoyant, the stinger supports the pipeline for substantial depth below the lay barge and thereby controls its curvature. From the point of departure from the stinger, the pipeline enters into the sag bend region and continue in this sag bend or catenary-like configuration to its touchdown position on the ocean floor. In order to control stresses in the unsupported portion of the pipeline, it is necessary to apply tension to the pipe continuously during the laying operation. This is normally accomplished by means of tensioners located on the deck portion of the lay barge.
In relatively shallow waters, the vertical descent of the pipeline being small, enables the development of large radii of curvatures for the pipeline as it comes off the lay barge into the overbend region and also permits a gradual curvature in the sag bend region as well. However, as the water depths increase, it is readily observable that the length of the unsupported pipeline also increases and this increasing length causes the pipeline to sag under its own weight. As a result, the combined stresses imposed on the pipeline in the sag bend region may exceed the allowable limits of the pipe material and result in permanent deformation of the pipeline. As can be fully appreciated a pipeline is capable of withstanding some bending but any bending of the assembled pipe, combined with other stresses acting on the pipe during installation, must not cause the pipe to be permanently deformed, buckled or ruptured so as to render the installed pipeline inoperative. Even under the modest depths achieved to date, over stressing of the pipe where the pipe departs from the stinger or in the sag bend region, have occcurred. While the traditional lay barge techniques has had limited success under North Sea conditions, to date this same technique has never been tested at the extreme subzero temperature nor at the extreme water depths, that will be experienced in the High Arctic regions.
Looking to contemplated Arctic channel crossings the first inclination is to attempt to extrapolate and modify conventional lay barge techniques to apply. However, many of the Arctic channel crossings are not free of ice cover or massive ice flows for a period long enough to allow a lay barge to navigate to the proposed crossing sites, let alone complete installation of a given crossing before the barge would become ice bound. In addition, the prospect of extending conventional lay barge techniques, to water depths in the order of 800 to 1200 feet is far from certain.
Previous mention has been made of the Arctic currents; one of the interesting considerations of Arctic currents is that they are in existence on a year around basis irrespective of whether the Arctic waters are ice covered or not. In addition, the currents have a tendency to be reversible over a short period of time. Under normal lay barge techniques, when a pipeline is laid in a channel where currents are prevalent, the barge can be manoeuvered in a yaw position so that as the pipe departs from the stern of the lay barge and from the terminal point of the submerged position of the stinger, the pipe in effect departs tangentially. This assures that the pipe is not over stressed in a lateral or horizontal direction due to the actions of the current on the pipe.
The present invention seeks to overcome the difficulties of operating upon water in Arctic conditions by utilizing the ice surface as the working surface. however it is not practical to adapt conventional lay barge techniques to an ice surface type operation. In order to facilitate a lay barge type technique on an ice surface it would be necessary to tow a barge-like sled across the ice laying the pipe through a slot or channel cut in the ice. Such a concept poses major problems. The weight of such an ice lay barge and the load that it would have to carry to support the pipe during the laying operation would probably cause the ice to fail. Of equal concern, unlike the floating lay barge, the stinger on which the overbend portion of the pipe is supported must pass through a relatively narrow slot or channel cut in the ice. Provided a slot of sufficient width could be excavated, the orientation of the stinger might be adjusted to avoid the aforementioned over stressing of the pipe at the point of departure from the stinger due to the lateral deflections caused by the current acting on the pipe. However in order to accomodate this orientation, an extremely wide ditch or channel would have to be cut into the extremely deep ice so that the orientated stinger would not become entangled with the bottom contour of the ice thickness itself. Even under these unlikely conditions, a further complication occurs, that of the lateral displacement of the pipe. Because the barge-like sled would be located on the top of the ice with the stinger projecting through the slot in the ice, it would be impossible to maneouver the barge to compensate for the large deflections due to the current and at the same time maintain the stinger in the channel.
Without this surface manoeuvering, the laying of the pipe along a required predetermined laying path, would be virtually impossible. The pipe in effect would have a zig-zag configuration in its final resting place on the bottom due to the effects of the reversing currents. The magnitude of these alignment deviations may be in the order of several hundred feet with the consequent danger of over stressing the pipe to the point of failure.
Obviously the standard technique of water borne lay barge methods for laying Arctic pipeline is impractical due to the predominance of the heavy ice. It is equally apparent that the utilization of a sled-like lay barge, operating from the surface of the ice, has serious deficiencies.
It has been proposed earlier to lay an underwater pipeline from an ice surface, by means of a barge-like device which was towed across the ice surface and carried a downwardly and rearwardly extending guideway, or stinger like element, which was heated in such a fashion that it cut a channel through the ice when the barge was pulled across the ice. The pipe was lowered to the bottom of the water down the stinger through the slot in the ice which it created.
The fact that water depths along some of the more favourable of proposed pipe line routes exceed 800 feet and at times approach 1,200 feet, together with the very thick ice encountered, makes this proposal impractical for high Arctic use. Clearly it would be unrealistic to melt a channel in very thick ice and equally unrealistic to attempt to lay pipe in regions where any cross currents were encountered. Furthermore, the simple stinger like arrangement of the prior proposal would be adequately support the pipe line for laying in great depths.